1. Field of the Invention
The invention relates to a transaxle apparatus having a housing which incorporates a hydrostatic transmission (HST) and a hydraulic actuator arranged outside the housing which can be supplied with hydraulic fluid from the HST. More particularly, it relates to a four-wheel-drive articulated working vehicle.
2. Related Art
A well-known articulated riding lawn mower has first and second frames which are mutually pivotally coupled at proximal ends thereof so as to turn relatively to each other around a vertically axial pivot steering operation (i.e., manipulation of a steering wheel). The first frame is equipped with a prime mover and a transaxle apparatus which supports first axles driven by power from the prime mover. The second frame is equipped with a working device such as a mower device, an operating section, and an axle casing that supports second axles freely rotatably.
In the Japanese Patent Laid Open Gazette 2000-270651, for example, is disclosed an articulated four-wheeled lawn mower, which includes as the first frame a rear frame and as the second frame a front frame. On the rear frame, a hydrostatic transmission (hereinafter, “HST”) is disposed, which transfers engine power to rear wheels supported by the rear frame. Moreover, in the rear frame is disposed a power take-off shaft, which receives power from a pump shaft of a hydraulic pump of the HST. The pump shaft revolves synchronously to the engine power output revolution. The revolution of the pump shaft is transferred to the mower device supported by the front frame.
Generally, as to each of vehicles having the above structure, while the first axles supported by the transaxle apparatus of the first frame (usually serving as a rear frame) is driven by the prime mover, the second axles supported by the axle casing of the second frame (usually serving as a front frame) revolve freely and not in driving association with the power for driving the axles of the first frame. Thus, the vehicle is a so-called two-wheel drive vehicle.
However, while the two-wheel-drive vehicle which drives only rear wheels exhibits superior steering performance, it lacks stability when working on a slope and roadability when running on a bad road. Further, if the vehicle is an articulated vehicle, the steering performance must be improved because the vehicle is bent at the coupling part of the frames. Moreover, the vehicle is difficult to bail out if it becomes stuck, such as in mud, etc.
For solving these problems, a four-wheel-drive design, which drives both front and rear wheels, is desirable for the articulated vehicle. The rear frame of the vehicle disclosed in the above document is provided with an HST and a power take-off shaft for transferring power to the working device. However, as mentioned above, since the power take-off shaft revolves synchronously with the revolution of the pump shaft, the rotary speed of the pump shaft is fixed as long as the engine speed is fixed. On the other hand, the rotary speed of the rear wheels, which are driven by the power output of the hydraulic motor, is changed variably by a running speed changing operation which adjusts the angle of a movable swash plate of the hydraulic pump. Therefore, the power take-off shaft for driving the working device cannot be used as a front wheel drive shaft. Even if another power take-off shaft for front-wheel-drive, whose rotation is synchronized with the power output of the HST for rear wheel drive, can be connected to the transaxle apparatus mounted in the rear frame, severe limitations exist for such an arrangement to infix additional mechanical transmission system between front and rear transaxle apparatuses, because the turning of front and rear frames must be permitted, as well as infixing the transmission system for the working device drive therebetween.